Apache MPM threadpool

Summary

Warning

This MPM is a developer playground and highly experimental, so it
may or may not work as expected.

This is an experimental variant of the standard worker MPM.
Rather than queuing connections like the worker MPM, the
threadpool MPM queues idle worker threads and
hands each accepted connection to the next available worker.

The threadpool MPM can't match the performance of
the worker MPM in benchmark testing. As of 2.0.39,
some of the key load-throtting concepts from the threadpool MPM have been incorporated into the worker MPM. The threadpool code is useful
primarily as a research platform. For general-purpose use and for any
production environments, use worker instead.